Okay, I think I understand now. I had been thinking of the Abstract piece
set as the default, not realizing that it was in fact a setting which had
overridden a previous default. So it makes sense that I can't override
it in the URL, especially now that I've gone and learned a bit about the
GET and POST methods of form submission.
Here's a related thought (related from a user's perspective, anyway).
When I enter my userid and view a game in which it is my opponent's turn,
the board is displayed, along with a message saying 'It is not your turn
yet', etc. But the board is shown from my opponent's perspective.
(Actually, a quick check of my current games seems to indicate that it is
shown from the perspective of the player to move [i.e. my opponent] in
Chess and Xiangqi, but from the perspective of the first [Black] player in
Shogi.) Perhaps it would be more user-friendly if specifying a userid
ensured that the board would be shown from that player's perspective.